Solid oxide fuel cells generally comprise a solid electrolyte such as yttria stabilized zirconia (YSZ) with a positive air electrode on one surface of the electrolyte and a negative fuel electrode such as a cermet of nickel and stabilized zirconia on the other surface of the electrolyte.
Results on solid oxide fuel cells (SOFC's) employing thin (.about.10 .mu.m) Y-stabilized zirconia (YSZ) electrolytes [1,2] have recently been reported. One of the main advantages of these thin-film-electrolyte cells is that electrolyte ohmic resistance is small even at reduced (600-800.degree. C.) operating temperatures T.sub.c [3]. Reduced SOFC temperatures should ease materials and processing problems associated with interconnection and gas sealing in SOFC stacks [4]. However, as T.sub.c is lowered below the typical value of .apprxeq.1000.degree. C., the rates of thermally-activated electrochemical reactions decrease, increasing electrode interfacial resistance r.sub.i and limiting the SOFC current density. In addition, the resistivity .rho. of perovskite air electrode materials increases with decreasing temperature.
Two general approaches have been used to reduce r.sub.i. First, the electrode and/or electrolyte compositions can be changed to provide improved catalytic performance. For example, (La,Sr)CoO.sub.3 (LSC) air electrodes on YSZ have been shown to provide improved performance over (La,Sr)MnO.sub.3 (LSM). The addition of a thin interfacial layer of a second electrolyte, such as Y-doped Bi.sub.2 O.sub.3 (YSB), can also lower r.sub.i [5]. Second, standard electrode-electrolyte combinations can be processed so as to maximize he three-phase boundary length. Electrochemical vapor deposition has been used to deposit YSZ within porous LSM electrodes to increase the contact area. Another means for increasing the contact area is via the use of a cermet electrode, e.g. Pt-YSZ or Ag-YSZ, where the additional contact area is within the cermet.
Cermet electrodes can also provide lower resistivity than ceramic air electrodes, especially at low temperatures. For Ag-YSZ cermets[6], for example, .rho. values were .apprxeq.30 times lower than those of LSC and LSM at 750.degree. C. There has been relatively little work done on cermets that contain noble metals and perovskite oxides, with only one preliminary report on Pt-LSM [7] for typical T.sub.c values of .apprxeq.1000.degree. C. We have observed that for SOFC's with Tc&lt;800.degree. C. Ag is an excellent candidate for cermets using perovskite oxide because of its good catalytic activity, high electrical conductivity, and relatively low cost.